This invention relates to aircraft landing systems and particularly to improvements in the low elevation angle accuracy of such systems. 2. Prior Art
Aircraft elevation guidance systems generally comprise a ground based transmitting system and an airborne receiving system. The ground base transmitting system includes a vertically scanning antenna which produces a narrow beam pattern of only a few degrees in elevation. This beam is swept in the vertical direction to illuminate an aircraft for only a brief interval during the sweep period. The elevation angle of the transmitting antenna is continuously measured and this information is transmitted with the beam to the aircraft. In one of the most commonly employed systems, the beam signal comprises a series of pulse pairs. The antenna elevation angle information is encoded in the beam signal by adjusting the time between the pulse pairs.
FIG. 6A illustrates the beam signal as received by the aircraft. The amplitude rises and then falls as the beam passes the aircraft producing an overall envelope which is the pattern of the transmitting antenna. The beam signal comprises a number of pulse pairs, illustrated in FIG. 6A as single lines such as the line 601. The individual pulses in each pair are shown in detail in FIG. 6B.
The time interval between the pulses in a pair, designated as T.sub.i and indicated by drawing numeral 603, is typically 12 microseconds. This time interval identifies the signal as the elevation guidance signal. The time between the pairs, designated T.sub.a and indicated by drawing numeral 604, provides angular information. Typically T.sub.a is 60 microseconds plus a number of microseconds equal to twice the elevation angle of the transmitting antenna expressed in degrees.
The transmitted pattern and method of transmitting elevation angle information in the newly proposed time reference scanning beam microwave landing system or MLS is shown in FIG. 7. The MLS system is a CW system in which a digital word 705 is transmitted first to identify the beam as an elevation beam. As the beam sweeps past the aircraft in one direction it produces a first beam pattern indicated by drawing numeral 701. In passing the aircraft the second time in the reverse direction, the beam produces another beam pattern 702. The time between the peaks of the two beams, T.sub.a, indicated by drawing numeral 704 represents the elevation angle of the transmitting antenna, and consequently, the elevation angle of the aircraft with respect to the landing surface.
In both the pulsed and the MLS systems, the aircraft receiving system calculates the elevation angle of the aircraft with respect to the surface as viewed from the transmitting antenna by determining when the center of the beam passed the aircraft and the value of the encoded elevation angle at that time. In both systems, the time of occurrance of the center of the pattern may be measured by halving the time that the beam signal exceeds a threshold.
FIG. 5A illustrates the transmitting antenna beam pattern and elevation angle at the center of the pattern. In this Figure, the antenna 501, transmits a beam with a pattern 502 at an aircraft 505. The center of the beam is represented by a line designated by drawing numeral 503 while the elevation angle .phi. is designated by drawing numeral 504.
Although there are differences between the pulsed and MLS systems in encoding angular information, both systems use a vertically swept beam which is subject to distortion caused by surface reflections at low elevation angles. This difficulty arises as the antenna is swept downward, bringing the edge of the beam to the earths surface. As the antenna continues to sweep downward, a portion of the transmitted signal is directed at the ground, producing a reflection which is received by the aircraft. This is shown in FIG. 5B where the beam pattern distortion 507, due to the reflections from surface 506, is evident. The distorted pattern causes an error to be produced in the aircraft receiving systems determination of the center of the beam. The result is inaccurate data at a critical point in the landing approach.
Prior art attempts to correct this have usually centered on installing large, costly antennas to narrow the beamwidth, an economically unsatisfactory solution where many existing installation would have to be retrofitted with large antennas.